Frequency modulator



June 17, 1941.

Filed Jan. 31, 1940 a Sheets- Sheet 1 C'A RR/ER PHASE FREQUENCY POWER SOURCE MODULATOR MUL TIPL/ER AMPLIFIER 1/ i :i 13 E:

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FREQ- M00. FREQUENCY BEA mva f CONVERTING CONVERTER OSCILLATOR 9 T &oErEcrm a (DETECTOR) v SYSTEM K 5 Bl 3 C k a p AUDIO FREauEA/cY INVENTOR MURRAY 6. C 058) BY ATTORNEY Pat ented June 17, 1941 FREQUENCY MODULATOR Murray G. Crosby, Riverhead, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1940, Serial No. 316,503

14 Claims.

This application concerns a new and improved means for and method of generating frequency modulation by means of a phase modulator which is subjected to inverse feedback in a manner which converts the modulation which normally would be phase modulation to frequency modulation.

In my United States application No. 136,578, filed April 13, 1937, I have described a frequency modulated oscillator which may be operated so that a phase modulation output may be obtained from the said frequency, modulated oscillator. This operation makes use of inverse feedback from the modulator output to the point at which the modulating potentials are applied. The invention of the present disclosure is similar, but is the converse of the system described in my prior application.

It is well known that frequency and phase modulation are so related that frequency modulation may be generated by applying the modulating potentials to a phase modulator through a correcting network which makes the amplitudes of the modulating frequencies inversely proportional to their frequency. The system of this disclosure utilizes that type of modulation except that in place of applying a network which predistorts the modulating potentials so as to cause a resultant frequency modulation, an inverse feedback circuit is set up which serves the same purpose. This method has the advantages normally obtained in the use of inverse feedback. These advantages include the reduction of hum and noises generated within the system and the reduction of distortion due to non-linearities.

In describing my invention, reference will be made to the attached drawings wherein:

Fig. 1 illustrates by block diagram the essential elements of my system and their general connections; s

Fig. 3 illustrates the arrangement of Fig. 1 also but in Fig. 3 certain of the circuits are shown more in detail; while Fig. 2 shows curves illustrating the operation of my system.

The circuit of Fig. l is a block diagram of the essential units of this system.. Certain parts of the apparatus employ principles known to the art. Other parts of the system are arranged and connected in a novel manner. The latter features are shown in detail in Fig. 3. Wave energy source I feeds voltages to be modulated to phase modulator 2 and thence to frequency multiplier 3. The output of frequency multiplier 3 is amplified in unit 4 and impressed on an antenna or other transmission medium. Part of the output of the frequency multiplier taken from 3, or if preferred, amplified output from 4 is supplied over lines it) to a frequency converter system 6. This frequency converter comprises a detector which, in conjunctionwith beating oscillator l, heteroclynes the phase modulated output of 3 or 4 down to a lower frequency which is more suitable for the operation of the frequency modulation converting and detecting circuits in unit 5. The apparatus included in 5 is for general purposes a frequency modulated Wave demodulator. The converting circuits of unit 5 may be of the back-toback type or may utilize only a single sloping filter in the manner described more in detail below. The function of the unit 5 is to convert the length modulations on the wave energy supplied from 6 to amplitude modulations and detect the same.

For phase modulation transmissiommodulating potentials are applied at jack 9 to excite transformer 8 connected to lines I I, and the connection to unit 5 is omitted or shorted by switch '3. Phase modulated wave energy is produced in 2, increased in frequency and modulation deviation in 3, amplified in 4 and transmitted. For frequency modulation transmission, switch S is opened and phase modulated Wave energy is supplied by radiation or lines ID to unit 6 and reduced in frequency therein due to the beating action between said modulated Wave energy and oscillations from unit 1. The phase modulated intermediate-frequency energy is supplied to 5. Note, for purposes of description, the wave energy supplied to 5 is said to be phase modulated, but, as will be seen later, due to the action of my novel system, the modulation will be changed and be of the nature of frequency modulation. When this phase modulated wave is received on the frequency modulation receiver of .unit.5, the signal potentials at the output vary in amplitude in accordance with their frequency. They have a frequency versus amplitude characteristic, as shown bythe line B of Fig. 2. The output of the frequency modulated wave demodulator in unit 5 is fed to a low pass filter l2. The purpose of this filter is to insert time delay in the feedback potentials so that the relative phases of the potentials from transformer 8 and demodulator 5 will be proper. Resistors I4 and I3 damp the low-pass filter and transformer 8 respectively. The normal connection of unit 5 to lines I l is such that the potentials at the output of 5 oppose those applied by transformer 8. The resulting potentials applied to modulator 2 then have a char- Since the potentials appearing at the output of unit aredue to the reception of phase modulation on a frequency modulation receiver, their amplitude characteristic will be as portrayed by r line B of Fig. 2. Their phase characteristic will be such that all of the frequencies will be shifted 90 degrees, or an odd multiple of 90 degrees, from the potentials applied to the phase modulator from transformer 8. Such a phase is not proper for inverse feedback because the resultant of the combination of the feedback and directly applied potentials would always be greater than the directly applied potentials. Consequently, time-delay unit I'Z, which may be a low-pass filter as shown, is inserted. This time-delay circuit adjusts the phase of the frequencies so that at the higher modulation frequencies the phase is near 180 degrees. If the phase were 180 degrees for all of the frequencies, the combination of the directly applied potentials given by line A in Fig. 2 and the feedback potentials given byline B, would result in a spectrum given byline C. However, since this out-o'f-phase condition is only approached at the higher modulation frequencies, the spectrum portrayed by line E in Fig. 2 is obtained. This spectrum has the desired characteristic of dotted line D of Fig. 2 which indicates the desired resultant characteristic of the modulation frequencies applied to the phase modulator to produce frequency modulation. The characteristic of line D is such that the audio-frequency output is inversely proportional to the audio frequency. As the degree of inverse feedback is increased, the characteristic of line E more nearly approaches that of line D so that practically complete compensation may be obtained and the output radiated by 4 will have the characteristics of a frequency modulated wave.

In Fig. 3 I have illustrated the essential feature of circuits included in units I, 2, and 5 of Fig. 1. The details of the circuits of the other units may beas known in the prior art and a showing thereof does not appear necessary to a clear and complete disclosure of my invention.

' The source of wave energy in unit I supplies voltage of carrier frequency'to an inductance 39 coupled to an inductance 32 in a circuit 34 tuned to the mean frequency of the wave energy to be modulated. Resistance R and condenser C form a phase shifter to effect phase shift in conjunction with the 180 degree shift of push-pull tuned circuit 34 so that the voltages of carrier wave frequency fed to the grids GI and G2 of the modulat'or ,40 and amplifier 42 are 135 degrees apart which is the optimum phase adjustment for this type of modulator. Phase shifter RC also effects an' amplitude reduction which is made such thatthe amplitude of the voltage of carrier Wave frequency fed to the modulated tube 4.0 is 0.707 times that fed to the unmodulated tube 42. The combined voltages appear in tuned circuit 50 which is connected to plates 44 and 46 of tubes and 42. The circuit 50 is tuned substantially to the carrier frequency.

Modulating potentials from source 9 are supplied by way of transformer 8 and leads H to the suppressor grid Al ,of .tube A0. .The voltages in tuned. circuit 50 are relatively modulated in amplitude at signal frequency. That is, the volt-' age supplied by tube 42 is unmodulated while that supplied by tube 40 is modulated. The voltages are phase displaced and combine to provide a resultant the phase of which varies in accordance with changes in the relative amplitudes of the voltages combined. This phase modulator is similar to the one shown in Fig. 3 ;of -my United States application #237,175, filed October 27, 1938, now United States Patent No. 2,238,249 dated April 15, 1941. The modulated wave is supplied from the tuned circuit 50 to the multiplier in unit 3.

The -modulated Wave, which has been heterodyned down for the frequency converter in unit fi is supplied to a limiter which may be a separate unit or includedinunit 5. This limiter comprises a tube EOhaVing a grid 62 and cathode 64 coupled to-the output of unit 6. Tube has two separate 7 triode sections as shown and these separate triodes may be in separate envelopes or in one envelope-a s shown.

The "triode system comprising cathode 64, grid 62, and anode 63 comprises an amplifier which amplifies the voltages supplied to the grid 62 and cathode 64 from the frequency converter in unitt. This input-is'conventional in construction comprising a grid resistor 59 and coupling condenser 6|. Between cathodes 65 and 64, which are connected together and ground, is connected a cathode coupling resistortt. The plate 63 of this first triode system is connected directly to a source of plate potential B+. The grid 68 of the second triode system is connected directly to ground andto the grounded end of resistance 55. The plate 61 of the second triode system is connected throughatuned circuit lllto the source of plate potential B+. A by-pass condenser or decoupling condenser BC is connected from ground to the common connection of the source of plate potential 'to the plates and '61. The anode 61 of the second triode system is coupled to the midpoint of the inductance -15 of the tuned circuit 16 through a coupling and blocking condenser 14. The other lead comprising the output of the second triode system is the ground connection'at the terminal of resistance '66 and at the terminals of resistances 86 designated more in detail hereinafter.

In this amplifier and limiter when the grid 62 is made positive increased cathode current is drawn through resistance 66 so-that the cathodes B4 and 65 of both triode systems are made more positive with respect to ground. Making the cathode 65 of the second triode more positive with respect to ground is equivalent to making the grid 68 of this second triode system more negative. Thus, a positive change on the grid 62 of the first triode system effects a resultant negative change on the grid 68 of the second triode system. This phase reversal causes the second triode system to effect negative grid limiting for the positive half cycles of the input Wave while the first triode system provides grid limiting for the negative half cycles. When grid 62 of the first triode system is swung negative, negative grid cut-off limits the change in cathode current caused ;by the input wave. When the grid of the first-triode system is swung positive, grid 68 of the second triode system is effectively swung negative until negative cut-off is reached for the, second triode system. A limiter such as comprised by tube 60 has been shown at Fig. -1 in my United States application o. 275,151, filed May 23, 1939.

The amplified and limited wave energy of reductance 13 coupled to inductance 15. The voltage induced by inductance 13 into inductance lvv reaches the girds 83 and 85 of tubes -82-and 84 in phase displaced relation. The voltages fed tothe mid-point of inductance l5 byway of condenser 14 reaches the grids 83 and.85 in like phase relation. The circuits including inductance 73, inductance 15, coupling condenser-14,:

tubes 82 and 84," etc. comprise a frequency discriminator circuit well known in the vart.

The detectors 82-and 84 are of the infinite impedance diode type and serve to rectify the. resultant voltages fed to their grids toproduce in the resistors 86 potentials characteristic of, changes in phase or frequency of the wave ener y,

from the output of the amplifierand' limiter from a selected mean frequency. The circuits and tubesare responsive to wave. length Variations .due to the fact that the voltagesinduced in 15., and applied in push-pull relation ,to the grids 83 and 85 change in phase with changesin wave length whereas the,voltagesysupplied in phase to the grids 83 and 85 by way of coupling condenser 14 are of substantially constant phase irrespective .of wave length changes of the voltage at the output of, the limiter and amplifier 60, The discriminator detector provides inthe output resistors 86 potentials which vary as a functioncf the wave length changesof the modulated energy supplied from the output of ampli fier and limiter Bl]. Amplitude variations in the voltages supplied to the discriminator circuit tend to cancel in the output of the balancedsystem as is well known. Similar systems have been shown in George United States Patent #2,035,745, dated March 31, 1936, and in Seeley United States Patent #2,l21,l03, dated June 21, 1938.,

. The said potential variations are amplified in amplifiers 9d and 92 which may be conventional and are impressed on the primary winding 9! of transformer I00, the secondary winding 93 of which is coupled to the low-pass filter circuit l2, the function of which filter circuit has been given above.

The frequency converting system comprising units 6 and I has the function of-reducing the frequency so that the frequency modulation converting circuits in unit 5 may. be eliminated by omitting units 6 and l and running lines directly to 5. Then additional audio-frequency amplification may be applied at the output of 5, or frequency converting circuits are used which were efficient at the frequency of the output of 3 or 4.

An amplitude limiter may be placed ahead of unit 5 or included therein as illustrated in Figs. 1 and 3 so as to remove the amplitude modulation unintentionally applied to the wave at any part of the circuit. However, the amplitude limiter may be eliminated and a single sloping filter substituted for the back-to-back arrangement of unit 5 illustrated in Fig. 3, so that the frequency modulation receiver in unit 5 will 'receive the undesired amplitude modulation as well as the frequency modulation components, and the inverse feedback effect will not only convert the phase modulation into frequency modula- That is, this-amplified andtion, but will also remove undesired amplitude modulation by opposing and compensatingthe same in'modulator.

Since frequency modulation may 'be received on a phase modulation receiver having a correction circuit in its output which makes the amplitude of the output proportional to the audio frequency, the frequency modulation receiver. of unit 5 may be replaced by such a corrected phase modulation receiver. The synchronized carrier required in the phase modulation receiver does not have to be filtered from the incoming signal, but may be obtained directly from source I.

What is claimed is:

1. Ina frequency modulator the combination of, a source of wave energy, a source of modulating potentials, a phase modulator connected with both of said sources to produce phase modulated wave energy, means for deriving from said phase modulator potentialswhich vary substantially in accordance with the frequency of the modulating potentials, and means for additionally modulating the phase of said wave energy in accordance with said derived potentials to convert said phase I modulated wave energy produced in said modulator to frequency modulated wave energy.

2. In combination, a signalling system com--v prising a source of wave energy to be modulated, a source of modulating potentials and a phase modulator connected with said sources in which I phase modulator said modulating potentials produce characteristic modulations of the phase of said wave energy, a demodulator normally-re,- sponsive to frequency modulated wave energy excited by said phase modulated wave energy and means for additionally modulating said wave energy in accordance with the modulation components in the output of said demodulator. I

3. In a frequency modulator the combination of, a source of wave energy, a source of modulating potentials, a phase modulator connected to said source of wave energy and with said source of modulating potentials for modulating the wave length of the former in accordance with the latter, means excited by wave length modulated energy from said modulator coupling said modulator to said source of modulating potentials whereby the modulating potentials are caused to vary substantially in inverse accordance with the frequency of the modulating potentials to thereby frequency modulate said wave energy in accordance with said modulating potentials.

4. In a frequency modulator the combination of, a source of wave energy, a source of modulating potentials, a phase modulator connected with both of said sources tomodulate the length of the wave energy by. the modulating potentials, means excited by wave length modulated wave energy from'said modulator for producing potentials which vary substantially in accordance with the frequency of the modulating potentials, and phase regulating means for impressing said produced potentials on said phase modulator whereby the frequency of said wave energy is modulated in accordance with said modulating potentials.

5. The method of operating a phase modulation system comprising a source of carrier Wave energy, a source of modulating potentials and a phase modulator coupled to both of said sources to phase modulate the carrier wave energy in accordance with the modulating potentials to produce frequency modulated wave energy which includes the steps of, deriving modulated Wave energy from said phase modulator, deriving potentials from said modulated wave energy the in proportion to the frequency thereof, modifying the phase of said potentials in accordance with their frequency and impressing said potentials on said signalling potentials used too phase modulate said carrier wave energy.

6.1 In a frequency modulator the combination.

of, a source of wave energy, a source of modulating potentials, a phase modulator connected'with both of said sources to modulate the length of.

the wave energy by the modulating potentials,

with the frequency of the modulating potentials,

and phase regulating means for impressing said produced potentials on said phase modulator whereby the frequency of said wave energy is modulated in accordance with said modulating potentials.

7. In a frequency modulation system, a source of wave energy to be modulated, a source of modulating potentials, a wave length modulator connected with both of said sources to modulate the length of the wave energy by the modulating potentials, a wave length modulated wave demodulator excited by wave length modulated wave energy from said modulator, a time delay networkcoupled to said demodulator, and a coupling between said time delay network and said wave length modulator to impress on said modulator modified modulation components from said demodulaton 8. A system as recited in claim 7 wherein a wavefrequency multiplier is coupled between said modulator and said demodulator to multiply the frequency of the wave energy used to excite said demodulator.

9. The method of operating a phase modulation system comprising a source of carrier wave energy, a source of modulating potentials and a phase modulator coupled to both of said sources to phase modulate the carrier wave energy in accordance with the modulating potentials to produce frequency modulated wave energy which includes the steps of, multiplying the frequency of modulated wave energy from the output of said phase modulator, heterodyning said modulated wave energy of multiplied frequency to a lower frequency, deriving potentials from said modulated wave energy of lower frequency the amplitudes of which vary substantially directly in proportion to the frequency thereof, modifying the phase of said potentials in accordance with their frequency and impressing said potentials on said signalling potentials used to phase modulate said carrier wave energy.

10. The method of operating a phase modulation system comprising a source of carrier wave energy, a source of signal potentials and a phase modulator coupled to both of said sources to phase modulate the carrier wave energy in accordance with the signal potentials to produce frequency modulation which includes the steps of, deriving modulated wave energy from said phase modulator, limiting the, amplitude of the derived wave energy, rectifying said amplitude limited modulated wave energy to produce poamplitudes of which vary substantially directly tentials of signal frequency the amplitudes of which vary substantially directly in proportion to'thesignal frequency, and impressing said potentials on said modulating potentials used to modulate said carrier wave energy. 7

. 11. In afrequency modulation system, a source of wave energy to be modulated, a source of modulating potentials, a phase modulator coupled to both of said sources to modulate the phase of, the ,wave ene gy in accordance with the modulating potentials, a frequency modulated wave demodulator excited by said modulated wave energy, a low-pass filter coupledto said demodulator and a coupling between the output of said low-pass filter and said phase modulator to impress on said phase modulator modified modulation components from said demodulator.

v12. The method of operating a phase modulation system comprising, a source of carrier wave energy, asource of modulating potentials and a phase modulator coupled to both of said sources to phase modulate the carrier wave energy in accordance with the modulating potentials to produce frequency modulated wave energy which includesthe steps of, deriving modulated wave energy from said phase modulator, deriving potentials from said. modulator wave energy the amplitudes of which vary substantially directly in proportion to the frequency thereof, modifying the phase of said derived potentials in accordance with their frequency, and impressing said derived potentials of modified phase on said phase modulator to additionally modulate said carrier wave energy.

Iii-The method of operating a phasemodulation'system comprising, a source of carrier wave energy, a source of modulating potentials and a phase modulator coupled to both of said sources to phasemodulate the carrier wave energy in accordance with the modulating potentials to produce frequencymodulated wave energy. which includes the steps of, multiplying the frequency modulated wave energy from the output of said phase modulator, heterodyning said modulated wave energy of multiplied frequency to a lower frequency, deriving potentials from said modulated waveenergy of lower frequency the amplitudes of which vary substantially directly in proportion to the frequency thereof, modifying the phase of said derived potentials in accordance with their frequency, and impressing said derived phase modified potentials on said phase modulator to additionally deviate said carrier wave energy. 7

14. Thewmethod of operating a phase modulation' system comprising, a source of carrier wave. ener y, a. source of signalling potentials and a phasev modulator coupled to both of said sources to phase modulate the carrier wave energy inaccordance with the signal potentials to produce frequency modulation which includes the steps of, deriving modulated wave energy from said phase modulator, rectifying said derived modulated wave energy to produce potentials of signal frequency the amplitudes of which vary-substantially directly in proportion to the signal frequency, and impressing said potentials on, said phase modulator to additionally deviate said. carrier wave energy.

- MURRAY G. CROSBY. 

